I have used them for over a year now and have no major complaints. My only issue is one month I got hit by their click discount and lost about 30% of my revenue. Outside of that now, they discount out of area clicks right away, and rarely do I see adjustments of more than a few percent. I think one month in the early days payment was a little late, but I have not had any late payments since then.
Can work really well on some sites, in some cases out peforming adsense and such substantially. However, a lot of sites you won't get any clicks at all, so it is worth playing around with.